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OTDR, the optical time domain reflectometer, is the most important investigation tool for optical fibres. It’s applied in the measurement of fibre loss, connector loss and for the determination of the exact place and the value of cable discontinuities. It’s the only device which can verify inline splices on concatenated fibre optic cables and locating faults.

To know how to use OTDR for the fibre investigations, first you should know the structure and working principle of OTDR equipment. When a short light pulse transmits into the fibre under test, the time of the incidence and the amplitude of the reflected pulses are measured. The commonly used pulse width ranges from nanosecs to microsecs, the power of the pulse can exceed 10 mW. The repetition frequency depends on the fibre length, typically is between 1 and 20 kHz, naturally it is smaller for longer fibres. The division by 2 at the inputs of oscilloscope is needed since both the vertical (loss) and the horizontal (length) scales correspond to the one-way length.

Besides, to use an OTDR successfully, you should also know how to operate the instrument. The following is about the experiences collected from some experienced people who use OTDRs during installation and for maintaining telecommunication networks.

Keep Connectors Clean

Before use OTDR, first, you should watch out if the connectors are clean. If it’s dirty, then clean it. Otherwise, it will make measurements unreliable, noisy or even impossible. What’s worse, it may damage the OTDR.

Check the Connector or the Patch Cord

Check whether the patch cord, the module, and the fibre under test are single-mode or multimode. To test the patch cord, activate the laser in the CW (Coarse Wavelength) mode and measure the power at the end of the patch cord with a power meter. This should be between 0 and – 4 dBm for most single-mode modules and wavelengths.

Set the Range

The range is the distance over the cable which the OTDR will measure. The range should be longer than the cable you are testing. For example, if your link is 56.3 km long, choose 60 km. For distances greater than approximately 15 km, make your first measurement in longhaul mode, otherwise use shorthaul.

Determine the Wavelength

Usually single-mode is set for 1310 nm or 1550 nm, and multimode is set for 850 nm or 1300 nm.

Averages of Noisy Traces

If the trace is very noisy, increase the number of averages. Usually 16-64 averages are adequate. To improve the signal to noise ration of the trace, the OTDR can average multiple measurements, but averaging takes time. So try to average over a longer time.

Realtime Mode

In this mode, you can modify parameters only if you stop a measurement explicitly. So it avoids you to erase a trace averaged over a long time by accident. You use realtime mode to check your connection, the quality of splices, and whether a fibre is connected. Start in automatic mode, then switch to realtime mode and select the most suitable parameters.

Adjust the Refractive Index

If you know the exact physical length of the fibre under test, you can measure the refractive index. Start with the refractive index 1.5000. Place a marker at the end of the fibre. Then select the refractive index function and adjust it until the displayed marker position is equal to the known fibre length. Then, the effective refractive index will be displayed.

Macrobending Loss

Single-mode fibres (1550 nm) are very sensitive to macrobending such as a tight bend or local pressure on the cable. It doesn’t always happen at this wavelength of 1310 nm. So characterise your link at both wavelengths.

OTDRs are invaluable test instruments. Maybe a small mistake will cause serious damage to this equipment. So before use it, you should better know it as detailed as possible to avoid any loss because of innocence and make full use of it in optical fibre events.

An optical time-domain reflectometre (OTDR) is an optoelectronic instrument used to measure fibre loss, the loss and reflectance of fibre splices, and to locate loss irregularities within the fibre. Now there are many types of OTDRs providing different test and measurement needs including very simple fault finders and advanced OTDRs for link certification. Then, how to choose the right one?

First, you should evaluate your needs. Installing or maintaining fibre? For simple maintenance, a simple or low cost OTDR is good. It’s easy to use, requires the lowest possible investment and some even provides total link loss and optical return loss values. For not very complex installation, you should choose a mini OTDR based on the following key parametres for your specific environment.

Dynamic Range

This specification determines the total optical loss that the OTDR can analyze; i.e., the overall length of a fibre link that can be measured by the unit. The higher the dynamic range, the longer the distance the OTDR can analyze. Insufficient dynamic range will influence the ability to measure the complete link length and affect the accuracy of the link loss, attenuation and far-end connector losses. It’s good to choose an OTDR whose dynamic range is 5 to 8 dB higher than the maximum loss you will encounter.

Dead Zones

Dead zones originate from reflective events (connectors, mechanical splices, etc.) along the link, and they affect the OTDR’s ability to accurately measure attenuation on shorter links and differentiate closely spaced events, such as connectors in patch panels, etc. There are two types of dead zones to specify OTDR performance:

Attenuation dead zone refers to the minimum distance required, after a reflective event, for the OTDR to measure a reflective or non-reflective event loss. Try to choose OTDR with the shortest possible attenuation dead zone to measure short links and to characterize or find faults in patchcords and leads. Industry standard values range from 3 m to 10 m for this specification.

Event dead zone is the distance after a reflective event starts until another reflection can be detected. If a reflective event is within the event dead zone of the preceding event. Industry standard values range from 1 m to 5 m for this specification. The event dead zone specification is always smaller than the attenuation dead zone specification.

Sampling Resolution

Sampling resolution refers to the minimum distance between two consecutive sampling points acquired by the instrument. This is a quite important parametre as it defines the ultimate distance accuracy and fault-finding capability of the OTDR.

Pass/Fail Thresholds

This parametre is also important because lots of time can be saved in the analysis of OTDR traces if you set Pass/Fail thresholds for parametres of interest (e.g., such as splice loss or connector reflection). These thresholds highlight parametres that have exceeded a Warning or Fail limit set and, when used in conjunction with reporting software, it can rapidly provide re-work sheets for installation/commissioning engineers.

Report Generation

If an OTDR has specialized post-processing software allowing fast and easy generation of OTDR reports, it can save up to 90% post-processing time. These can also include bidirectional analyses of OTDR traces and summary reports for high-fibre-count cables.

To choose a right OTDR for your test application, you should better consider the above factors. FS offers YOKOGAWA AQ1200A, EXFO AXS-110-23B-04B OTDR, etc,. with great accuracy, measurement range and instrument resolution. There must be one suitable for you and helpful to maximize your return on investment.

Originally published at http://www.articlesfactory.com/articles/communication/how-to-choose-a-right-otdr.html

As the use of fibre in premise networks continues to grow, so do the requirements for testing and certifying it. An optical time-domain reflectometer (OTDR) is an electronic-optical instrument used to characterize optical fibres. It locates defects and faults, and determines the amount of signal loss at any point in an optical fibre by using the effects of Rayleigh scattering and Fresnel reflection. By sending a pulse of light into a fibre and measuring the travel time (“time domain”) and strength of its reflections (“reflectometer”) from points inside the fibre, it produces a characteristic trace, or profile, of the length vs. returned signal level on a display screen (as shown in the following picture). This article will describe the key specifications that should be considered when choosing an OTDR.

When choosing an OTDR, it is important to select the specific OTDR performance and features according to the required specifications listed below.

Dynamic Range

The dynamic range of an OTDR determines how long of a fibre can be measured. The total optical loss that an OTDR can analyze is mainly determined by the dynamic range. The dynamic range affects the accuracy of the link loss, attenuation and far-end connector losses. Thus, having sufficient dynamic range is really important. The manufacturers specify dynamic range in different way. The higher the dynamic range, the longer the distance an OTDR can analyze.

Dead Zones

Dead zone refers to the space on a fibre trace following a Fresnel reflection in which the high return level of the reflection covers up the lower level of backscatter. To specify an OTDR’s performance, it is important to analyze the dead zone and ensure the whole link is measured. Dead zones are characterized as an event dead zone and an attenuation dead zone. Event dead zone refers to the minimum distance required for consecutive reflective events to be “resolved” (for example, to be differentiated from each other). Attenuation dead zone refers to the minimum distance required, after a reflective event, for the OTDR to measure a reflective or non-reflective event loss.

Resolution

There are two resolution specifications: loss (level), and spatial (distance). Loss resolution is the ability of the sensor to distinguish between levels of power it receives. When the laser pulse gets farther out in the fibre, the corresponding backscatter signal gets weaker and the difference between backscatter levels from two adjacent measurement points becomes larger. Spatial resolution is how close the individual data points that make up a trace are spaced in time (and corresponding distance). The OTDR controller samples the sensor at regular time intervals to get the data points. If it takes readings from the sensor very frequently, then the data points will be spaced close together and the OTDR can detect events in the fibre that are closely spaced.

Pass/Fail Thresholds

This is an important feature because a great deal of time can be saved in the analysis of OTDR traces if the user is able to set pass/fail thresholds for parameters of interest (such as splice loss or connector reflection). These thresholds highlight parameters that have exceeded a warning or fail limit set by the user and, when used in conjunction with reporting software, it can rapidly provide re-work sheets for installation/commissioning engineers.

Post-Processing and Reporting

Report generation could be another major time saver. For example, some OTDRs with specialized post-processing software allow fast and easy report generation, which might reduce the post-processing time up to 90 percent. These reports also include bidirectional analyses of OTDR traces and summary reports for high-fibre-count cables.

Your Applications and Users

Some OTDRs are designed to test long distance optical fibres and some others to test short distance optical fibres. For example, if you are to test premises fiber networks where short distance optical fibres are installed, OTDRs designed for testing long distance optical fibres are not suitable. Besides, knowing your users and the time it will cost is also necessary. Because some types of OTDRs are easy to use and some others are complicated to set up.

When selecting an OTDR, you’re supposed to take all the above factors into consideration. Fiberstore supplies a wide range of OTDRs available with various fibre types and wavelengths (including single-mode fibre, multi-mode fibre, 1310nm, 1550 nm, 1625 nm, etc). They also supply OTDRs of famous brands, such as JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so on. OEM portable and handheld OTDRs are available as well.

The Fibre Fault Locator (VFL) is an essential tool for every Fibre Termination Kit. It is like the continuity tester. The VFL is not one of the least expensive tools in your tool kit. It will allow you to quickly identify breaks or macrobends in the optical fibre, and identify a poor fusion splice in multimode or single-mode optical fibre.

The big difference between the VFL and the continuity tester is the light source and optical output power of the light source. The VFL typically uses a red (635-650nm) laser light source. The optical output power of the laser is typically 1mW or less. Because of the high optical output power, you should never view the output of the VFL directly.

The Visual Fault Locator is available in different shapes and sizes. Some may look like a pen. Others may be built into an optical time domain reflectometer (OTDR), and some may look like a small test equipment box. There are two types of VFLs: contact and non-contact. With a contact VFL, the optical fibre under test will make contact with the VFL. However, with a non-contact VFL the optical fibre under test will not touch the VFL.

Unlike the continuity tester, the VFL is not limited to testing multimode optical fibres 2km or less in length. The VFL can be used to verify continuity of multimode or single-mode optical fibre longer than 2km. Due to attenuation of the 635–650nm laser light source by the optical fibre, macrobends may not be detectable beyond 1km in multimode optical fibre and 500 metres in single-mode optical fibre. The same holds true for finding breaks in the optical fibre through the jacket of the fibre-optic cable.

How to Use Visual Fault Locator

As with the continuity tester, the first thing you will need to do is clean the connector endface and inspect it with a microscope. If the endface finish is acceptable, the VFL can be connected to a Optical Fibre Connectors should not be viewed directly during this testing.

The VFL fills the core of the optical fibre with light from the laser. The light from the laser escapes the optical fibre at a break or macrobend. The light escaping from the optical fibre will typically illuminate the buffer surrounding the optical fibre. Macrobends are not always visible through the jacket but are typically visible through the buffer. Breaks may be visible through the jacket of the fibre optic cable depending on jacket color, thickness, number of optical fibres in the cable, and amount of strength member.

The VFL and the fibre OTDR work hand in hand with each other when it comes to locating breaks in an optical fibre. The OTDR can provide the operator with the distance the break. The VFL allows the operator to see the break in the optical fibre.

Fibre optic cables are not the only place where the optical fibre may break. The optical fibre may break inside the connector or connector ferrule. Unless the optical fibre is broken at the endface of the connector, it is not visible with a microscope.

Usually, students connect cables that look great when viewed with the microscope but fail continuity testing. When this happens, the hardest part is determining which connector contains the break in the optical fibre. Without a VFL in the classroom, students would have to cut the cable in half and use the continuity tester to identify the bad connection.

The VFL will often identify the bad termination or connector. Looking at the photograph, you can see VFL illuminating the break in the optical fibre. The output of the VFL is so powerful that it penetrates the ceramic ferrule.

The visual fault locator can be used to test the continuity of an optical fibre in the same manner. The first step when using the continuity tester is to clean and visually inspect the end face of the connector before inserting it into the continuity tester. After the connector has been cleaned and inspected, you need to verify that the continuity tester is operating properly. Turn the continuity tester on and verify that it is emitting light.

The visual fault locator also can be used to locate a macrobend in an optical fibre. However, macrobends do not allow nearly as much light to penetrate the buffer and jacket as does as break in the optical fibre. Locating a macrobend with the VFL may require darkening the room.

Macrobends and high loss fusion splices appear the same on an OTDR trace. The VFL allows the identification of a high-loss fusion splice.

Network test and monitoring solutions provider Fluke Network recently unveiled a new line of Versiv cable certification tester, which greatly improved the test time and accuracy as well as simplified the testing setup, planning, and reporting. This new line has an interchangeable module for copper, fibre and Optical Time Domain Reflectometer (OTDR) testing with new software innovations.

The new Versiv family addresses the entire certification lifecycle and lower instances in which mistakes are made, thereby increasing the amount of installations that can be performed. The new ProjX management system provides the overall umbrella for the Versivfamily of capabilities. It enables team leaders to set up test parameters to work across multiple jobs and media, as well as accelerates the planning and setup of projects by enabling technicians to capture consistent test parameters across an entire job. Test can be also performed across the entire job or switching between jobs by checking the different projects stored within the Versiv tester.

Versiv system’s interface is designed for ease of use in global ISO Level V test compliance applications. The tester is interfaced with an intuitive touchscreen which will give the detailed project analysit. If technicians run into a problem that can’t be addressed immediately, they can create a to-do list so it can be assessed by a more experienced technician. The new Versiv Cabling Certification family lets installer perform:

Certify two fibres at two wavelengths in both directions in just three seconds

“When doing cabling installation and certification, the difference between having a job be profitable versus a loss, is often times just a few percentage points,” said Jason Wilbur, vice president and general manager of the datacom cabling installation business unit at Fluke Networks, via a press release. “In 2004, we defined the certification market with the introduction of our industry leading tester, the DTX, which was focused on certification testing speed. But today’s challenges have changed and our customers must improve their agility and reduce errors when working across multiple mediums, codes, and projects. The Versiv family is razor focused on helping our contractors profitably manage the complexities that are now part of their new normal.”

This new Versiv Cabling Certification family will shortly be available at FS.COM.

eHuawei announced that they have successfully developed the industry’s largest spectral built-in optical time domain reflection tester, which maximum support is 1:64, through the network simulation, its precision is up to 5 metres. The technique breakthrough marked the eOTDR technology has reached FTTHPON network commercial networking requirements.

Embedded Optical Time Domain Reflectometre, which is short for eOTDR. It’s the utilisation of scattering light in optical fibre transmission and precision instrument, is mainly used for optical fibre quality detection and fault location, etc. Traditional external OTDR test system in PON FTTH network maintenance and fault detection, the need to change the ODN physical optical fibre connection, the system has high cost, difficult to implement.

With the development of FTTx, optical fibre developed quickly. Operators increase investments for fibre optic network year by year. How to manage cables management quickly and efficiently, to reduce OPEX, become the urgent demand of operators FTTx network construction.

Huawei eOTDR prototype through the built-in OTDR in OLTPON optical module, which can judge the fibre’s physical connection. At the same time, the built-in OTDR module and ordinary optical module size is consistent. Operators will place ordinary optical module with built-in OTDR optical module, but can not change the FTTx fibre physical network, also do not need ONT extra coordinate positioning, to avoid the external engineering of OTDR test, shorten the time needed for a fibre fault location, reduce the fibre optic fault management costs.

The industry mainstream manufacturers provide 1:8 eOTDR product ratio, after many years of technical research and experimental verification, the breakthrough to develop the 1:64 eOTDR prototype, covering the mainstream of FTTHpatch cablesconstruction scene, marked the eOTDR technology realized, breakthroughs from lab scale to commercial technological.